1. Field
The methods and systems disclosed herein relate to generating bremsstrahlung with beams of electrons having high intensity and high areal densities that enhance the photon flux in a narrow cone at forward angles while suppressing the radiation at large angles.
2. Background Information
The use of bremsstrahlung as a source of photons may find application in many modalities that require a large photon flux spread over a large area. Such an application may use a thick target such as tantalum, tungsten or another high-Z material that has a relatively small radiation length and efficiently converts the kinetic energy of an electron into radiation energy. The thick target not only may provide efficient radiation, it also may spread the electron beam in angle via multiple scattering which in turn may help to spread the radiation pattern over angles much greater than the natural angle of thin target bremsstrahlung given by ˜1/γ, where γ is the ratio of the electron rest mass to the total electron energy, mc2/E. In such applications the electron beam may often be swept over the high-Z radiator to further spread the radiation pattern. Practical aspects such as the need to cool the targets may limit the total electron beam power and its areal density and for high intensities continuous operation at one beam position may not be possible.
In other applications, by contrast, it may be desired to use a bremsstrahlung beam confined to a narrow cone in order to define a small region of space to be irradiated. In this case the intensity of the beam usually may be desired to be approximately uniform over the narrow aperture of the cone. Any radiation outside the cone may not be useful. In fact, shielding may be required to prevent the interference of signals from other regions, to prevent background in detectors, and also for reasons of personnel safety. In such situations the use of thinner bremsstrahlung targets than those discussed above may be advantageous because less radiation is generated in the angles where the radiation is not useful.
In these situations multiple scattering plays an important role as the physical phenomenon that allows the angular distribution of the bremsstrahlung to be broadened beyond 1/γ. As an example, for a beam of electrons of 10 MeV kinetic energy (10.51 MeV total energy, E), the natural angle of thin target bremsstrahlung (mc2/E) is approximately 0.049 radians or 2.7 degrees. As a bremsstrahlung target is increased in thickness the multiple scattering soon becomes considerably larger than 2.7 degrees and the intensity at zero degrees no longer increases linearly with thickness. In fact the intensity almost saturates with increasing thickness. The bremsstrahlung beam simply grows to fill a wider angular region as the target thickness is increased. In addition the energy of the electrons is decreased by the ionization losses and in turn this affects the photon spectrum that is produced, in particular the intensity at the highest energies compared to the intensity at lower energies. Those photons beyond the desired angle not only are useless for such applications, they can provide deleterious effects and need to be removed.
U.S. Pat. No. 3,999,096 to Funk et al. teaches the use of a layered multi-element bremsstrahlung source using a high-Z, low-Z, high-Z layered structure. The first layer is a thick high-Z layer for bremsstrahlung production from an energetic electron beam, the second layer is a thick low-Z material for complete stopping of the electron beam, and the final layer is another high-Z material for absorbing low energy photons.